Email auto-discovery means a client email is able to automagically find out about what ports and security options to use, based on the mail server URL. It can help simplify the tedious / confusing task of adding own's email account for non-tech savvy users.
Basically, email clients will search for auto-discoverable settings and prefill almost everything when a user enters its email address ❤️
There exists autodiscover-email-settings on hub.docker.com which provides IMAP/POP/SMTP/LDAP autodiscover capabilities on Microsoft Outlook/Apple Mail, autoconfig capabilities for Thunderbird or kmail and configuration profiles for iOS/Apple Mail.